Your war of agressions will not be tolerated

Please.

You should know by now that politicians see events through their own slightly off color and out of focus glasses.

Heh. Very true.

It is worth noting that when you declare war to honor an alliance, the game treats you as the aggressor - it doesn't care why you declared war, only that you did, not the other race. This can be significant if you have treaties with the race you're declaring war on, as you'll be blamed for breaking them and considered untrustworthy.

Incidentally, when an AI says that declaring war on you is 'lucrative' or something similiar, it means they've been paid to attack you - most likely by the Drath.

Also, you will be considered more aggressive if you totally wipe out your enemy (as you did when you took their one and only planet).

I always try to leave them several planets when I end the war with them. It makes me appear less aggressive, even if those remaining planets are eventually flipped by my influence. (I try to make sure they have no planets left with overlapping influence)

In the case of a single planet major civ it might be better to simply destroy any ships and starbases, take resources etc. for a time. Then you can offer them a peace treaty. If you do it long enough you will gain more in the end by keeping them subdued while you are researching and building up your forces. Plus you look better to the other civs by not committing genocide.

I tried negotiating peace when they have only planet left.  Often I get the message that the ambassador will not see me because of the recent hostilities.

If an AI is about to be wiped out, I think they SHOULD agree to speak to me I mean when their defending ships are gone from their last remaining planet and I have a troop transport that is about to invade what can be more important ?

It's like I have to wait several turns doing nothing and possibly losing a good strategic position just so I can speak to the ambassador.

In the case of the Drath, he only had 1 planet left,  half of his population was gone from it.  He had no one defending it still he refused a peace for peace treaty.   He was expecting me to pay him something in order to get the peace treaty (All I had was some military tech that I didn't want to give him).   My diplomatic skills were higher than his so it makes no sense to me that he would be so demanding in his peace treaty.  It seems to me that a race about to be wiped out is in no position to make demands.

Besides how can it be genocide ?  In the game it states that what is shown as our population are the people on the planet that consider themselves our citizens (the higher the approval the more people declare themselves our citizens).  This means that whoever is defending the planet against invasion are the people on the planet who consider themselves citizens of that empire.  To me that means that when a planet is conquered, what remains are the non-citizens who didn't fight against the invasion.  (Except when spore ships are used)

I don't think approval rating for a planet has anything to do with how many fight when being invaded. That number is strictly a percentage of the population.

And I'm sure you can do the math. If you send 1B troops and half are lost, your population on the newly taken world will be .5B. The only way to make that number larger would be to use Information Warfare to get some defectors before the actual fighting starts (I don't know about Spore Ships, I only have DL). So while you can make up a story about how many survive, they will never be counted as part of your population. They were, are and always will be invisible.

As to why the AI won't talk peace when they are down to a single planet, well, maybe they consider it pointless and would rather go out fighting.

I think the basic concept behind the "population is just citizens" thing is to explain why populations grow at such an unrealistically fast rate - it's not just new people being born, but non-citizens becoming citizens. So in a way high approval does mean your number of citizens grows (up to a point).

Of course, this has some story holes of its own, but whatever.

If an AI who's clearly losing refuses to make peace, often it means that they hate your civilization (which can be seen on the "Viewpoint" tab on the diplomacy screen), usually because of multiple wars or surprise attacks. Not sure why that would be the case in your game though.

Incidentally, I've noticed that the Arceans are one of the races most prone to '"acting to stop aggression". Sometimes it's fairly justified, sometimes it's just a thinly disguised attempt to slice off a bit of a dying empire (like in my current game).

I just played a new game with the same opponents.  This time I left my opponent a planet and the other races were less hostile towards mine.   The only exception was near the end when a race I had created based on the Altarians was in the process of conquering a cluster of minor races.  If they had taken those planets it would have carved in twain my current empire.  So I waited till the minor race's defenders were gone and sent troop transports to take them.  (There was no law requiring me do declare war before hand).

Taking those minor races planets did give me the "Tendency to conquer others label" in the report.  Next time, I may give them a planet that will flip back before invading them.  I noticed that invading a minor race that has treaties with me will not affect my reputation the way that it does if a major race has a treaty with me.  ( I don't have a treaty with them, I just exchanged some tech for their treaties.  Near the end every major race had either an economic or research treaty with me (or both in some cases).

I tried one tip someone suggested in another discussion but that didn't work.  The tip consisted of placing a scout near an alien planet (Korx) and then converting it to a tiny hull with a weapon and no engine and then giving it to another race (Torian).  The idea being that the Torian wouldn't be able to move the ship out of the sphere of influence of the Korx and the Korx would eventually declare war on the Torian.    The Torian was somehow able to move that ship in orbit around his planet despite the fact that it didn't have an engine.   Other races I tried this trick on did the same.  I had no sensors in that area so I couldn't see the ship move (or the "tow truck" arrive).

In this game when someone paid an alien race to attack me, the first time I bribed the Drengin and Korath to attack the Drath as they had bad relations with them anyway.  The second time, someone paid the Carinoids to declare war on me.  So I invaded the Drath homeworld without declaring war and left them their secondary class 4 planet that eventually flipped. 

In the same game, the Korx tried to bully me into giving them credits for their "peace keeping efforts" (they weren't at war with anyone).  The first time I paid it, but they came back for more within a few turns.  The second time I noticed they only had 1 warship so I traded for that ship, then I asked a tribute for the same amount.  (Which they refused to pay).

Hulls typically have a speed of 1 built in, so even with no added engines they can still move slowly.

You could even mod hulls with any speed you want.

This will not work as minor races are immune to cultural conquest.

Just so you know, I saw this behaviour yesterday.  A race I paid to go to war with the Yor got war declared on them by the Yor's economic/research partner, with a message to me about stopping the war of aggression.  The poor guys got reduced to just two colonies before I put a nice bribe in the hands of the Yor's partner to backstab the Yor.

I don't know if it's the difficulty setting or my bribing a race to go to war that set it off, but I sure haven't seen it before now.

The gifting of a warship w/o engines to initiate wars between opponents was an idea I suggested.  It appears that a better approach may be to gift a troop transport (even with very low troops).  The downside is that the gift receiver may need to have Invasion tech.  That is, I am unsure if one can invade with a troop transport w/o having the tech.  W/o the tech, the troop transport gift might not be seen by the other enemy as a threat.

I suppose one test would be to play a game, avoid getting the invasion tech, and then try to trade for an AI troop transport.  Once in possession of said transport, attempt an invasion.  That does not seem very easy to set up, however.

I like the trick of giving an about to be conquered foe a low pop planet deep in one's own Influence zone just before invading their last planet.  I have used the work-around of carefully choosing which single planet to leave them before peace.

Many times, the AI empire will surrender (not always to me!) when down to their last few planets.  I suppose I could turn-off surrenders, but I like the unpredictability.  Also, turning off surrenders guts the Temple of Empathy of any value, and I doubt the AIs are smart enough to recognize that.

It will work, had quite a number of games where this did happen. Think of it the same way you gift/get a warship that has weapons on it although you are not in possession of the techs. You still can use the ship 100%.

Very good tactic in order to avoid a diplo-penalty. Empty the planet beforeahead (manually), see it flip fast then.